That is incorrect - there are 2 water drainage systems in the UK. Surface water / rain goes into the surface system and that flows freely into the water table untreated (rivers, lakes etc). It is not designed for dirty water.

The sewage system is totally separate - that is for contaminated water (toilets, sinks) and that goes to sewage treatment plants. It should be treated before it is released into the freshwater system.

So yes, it IS polluting the fresh water by putting things into the rain drainage system.